From Netcraft's latest web survey: "Microsoft adds 2.4 million sites this month, pushing the total number of sites running on Windows servers past 40 million, and helping Microsoft improve its market share by 1.01% to 32.8%. The open source Apache server has an increase of 556k, and slips back 1.11% to 52.65%. Google gains 592k sites this month, and now has 4.35% share. In active sites, Apache is now at 49.98% share, less than 14.5% ahead of Microsoft. While that's still a considerable lead, Apache had a 33.4% advantage at this time last July, meaning MS has cut its deficit in half in the past 12 months."

"PLEASE NOTE: The statistics provided should NOT be used to compare the overall security of products against one another. It is IMPORTANT to understand what the below comments mean when using the statistics, especially when using the statistics to compare the vulnerability aspects of different products."

Apache is equivalent of IIS in security. You just need to read the ENTIRE report to understand why.